1. Field of the Invention
The invention relates to an optical system for accessing a storage medium.
2. Description of the Related Art
FIG. 1 schematically shows a conventional optical pickup head for accessing a compact disc (CD) and a digital versatile disc (DVD). The conventional optical pickup head 10 has two laser diodes 1a, 1b emitting two beams with different wavelengths, two holographic optical elements (HOE) 2a, 2b, a beam splitter 3, a collimating lens 4, a mirror 5, and an objective lens 6.
As shown in FIG. 1, the conventional optical pickup head has two optical paths L1, L2, wherein one optical path L1 starts from the laser diode 1a to the DVD, and the other optical path L2 starts from the laser diode 1b to the CD. The laser diode 1a emits a beam with a wavelength of 650 nm, and the beam passing the HOE 2a is incident on the beam splitter 3. The beam passes through the beam splitter 3 and is incident on the collimating lens 4. Thus, the collimating lens 4 emits a collimated beam, and the collimated beam is reflected by the mirror 5. The collimated beam reflected by the mirror 5 is projected on the DVD 7 by the objective lens 6. Further, the DVD 7 emits a signal beam, and the signal beam following the foregoing optical path enters a detector (not shown) adjacent to the laser diode 1a, whereby the detector converts the signal beam reflected from the DVD 7 to an electric signal.
Referring to FIG. 1, another laser diode 1b emits a beam with a wavelength of 780 nm, and the beam passing the HOE 2b is incident on the beam splitter 3. The beam splitter 3 reflects the beam from the HOE 2b, and then the beam is incident on the collimating lens 4. The collimating lens 4 emits a collimated beam, and then the collimated beam is reflected by the mirror 5. The collimated beam reflected by the mirror 5 is projected on the CD 7 by the objective lens 6. Further, the CD 7 emits another signal beam, and then the signal beam following the foregoing optical path enters another detector (not shown) adjacent to the laser diode 1b, whereby the detector converts the signal beam reflected from the CD 7 to an electric signal.
However, the lengths of the two optical paths in the conventional optical pickup head are too long. Referring to FIG. 1, the optical path L1 has a length of 22.732 mm.